Diseases caused by organisms of the phylum Apicomplexa include malaria, toxoplasmosis and coccidiosis.
Malaria is a tropical disease, spread by mosquitoes from person to person, that exacts a devastating toll in endemic regions, especially Africa, where it claims 1 to 2 million lives each year. The deaths occur primarily among young children and pregnant women—vulnerable populations for whom therapeutic options are limited. These options are even more restricted in the current landscape of widespread drug resistance in the Plasmodium parasites that cause malaria. Together with an increasing incidence of malaria worldwide, there is an urgent and unmet need for new drugs to prevent and treat malaria, an infection that causes clinical disease manifestations in 300 to 500 million people each year.
Malaria is a worsening global health problem. The incidence of malaria continues to increase worldwide, due in part to the emergence of drug resistance but also due to global warming. Initially observed in the late 1950's and early 1960's in South America and Southeast Asia, chloroquine-resistant Plasmodium parasites that are associated with the most virulent form of malaria, cerebral malaria, have now spread to all malarious regions of the world. Varney et al. (1994) (1997) and others report a strong correlation between cerebral malaria and neuropsychiatric symptoms, such as poor dichotic listening, ‘personality change’, depression, and, in some cases, partial seizure-like symptoms. The tropical neuralnesia resulting from the legendary malarial fevers is well known in the endemic areas and has been documented throughout history.
Chloroquine replacement drugs are urgently needed to treat and prevent malaria. The endoperoxides, like artemisinin (derived from a Chinese herbal remedy extracted from the wormwood plant) are being used in other parts of the world for malaria therapy. However, the use of this remedy is limited by reports of ototoxicity and neurotoxic effects of the endoperoxides. More recently, severe reproductive toxicity in female rats has been reported in animals treated with artesunate and its active metabolite, dihydroartemisinin. These findings are mirrored in reports by others in several different animal models.
While the great panacea for malaria therapy would be the development of a long-lasting vaccine, the failure of the SPf66 vaccine and unrealized potential of newer multi-component DNA vaccines, combine to indicate that a vaccine is a long way from reality. As a result, the need continues to exist in the medical field for the development of safe, inexpensive anti-parasitic agents, especially agents that are useful against multi-drug-resistant organisms such as P. falciparum and P. vivax. 
Toxoplasmosis, caused by Toxoplasma gondii, is a leading cause of birth defects and it is estimated that the health care costs due to toxoplasmosis are roughly 5 billion dollars each year in the United States. In addition, there are researchers who believe that latent toxoplasmosis infections may underlie certain mental illness conditions including schizophrenia.
Hans Andersag is well known for the discovery of chloroquine (resochin) in the 1930's. He was also connected with the discovery of “endochin”, a compound that elicited great interest among Bayer scientists because of its efficacy in treatment and prevention of malaria in a bird model (P. cathemerium/canary) of the disease. In subsequent work summarized by Kikuth and Mudrow-Reichenow, Steck, and Wiselogle, endochin also demonstrated efficacy in treatment and prophylaxis against P. gallinaceum in the chick and P. lophurae in the turkey. Kikuth further reported that endochin exerted gametocidal action against male gametocytes undergoing exflagellation in finches infected with Haemoproteus, a closely related member of the Apicomplexa. Despite these unique and desirable qualities, endochin's antimalarial potential was never realized because it failed to cure malaria infections in subsequent experiments in mammalian species ranging from mice to non-human primates (Rhesus monkeys).